Carbide fuels have been considered for advanced nuclear reactors because of properties such as high thermal conductivity, high fuel-atom metal density and high stability under most reactor conditions. For advanced reactor designs aimed at space applications, uranium monocarbide (UC), uranium dicarbide (UC.sub.2), and uranium-zirconium carbide are seen as potential fuels. Considerable prior work has examined various processes for preparing UC and fabricating UC fuel pellets. For example, U.S. Pat. No. 3,578,610 to Johnson et al. describes a process for fabricating stoichiometric uranium monocarbide wherein the uranium monocarbide is placed upon tantalum carbide during sintering at temperatures from about 3500.degree. F. and about 3800.degree. F. However, there has been considerably less work reported on the preparation of UC.sub.2 and the fabrication of UC.sub.2 fuel pellets.
Most of the work on fabrication of UC.sub.2 pellets by conventional techniques has generally involved the blending of high purity uranium powder and graphite and heating in an arc melter under an inert gas to form UC.sub.2, followed by milling the UC.sub.2, cold pressing the particles and sintering to obtain the pellets. Such a process can generally only attain densities of about 80 percent theoretical density. High density products approaching about 97 percent theoretical density have been described in Carbides in Nuclear Energy, Vol. 1, edited by Russell (1963). High density, large grained fuels are desirable for the operation of space nuclear reactors. Prior art processes have been unable to achieve high densities of greater than about 97 percent theoretical density, to achieve large grain sizes of greater than about 30 microns in diameter, or to achieve both high densities and large grain sizes.
It has been previously known to use various materials as sintering aids. For example, J. Nuclear Materials, 89, 296-315 (1980), by Pickels et al. entitled "The Sintering of Uranium Carbide and Uranium-Plutonium Carbide, and The Role of Nickel as a Sintering Additive", describes the use of nickel in a sintering process, although without the same mechanism of densification as in the present process. Further, in the Pickels et al. process, the sintering aid, i.e., the nickel additive, is essentially co-blended during the process and remains in the final composition. Thus, the sintering aids such as nickel remain in the final product as an impurity.
Accordingly, it is an object of this invention to provide a uranium dicarbide material having a high theoretical density, i.e., greater than 97 percent of theoretical density.
It is a further object of this invention to provide a uranium dicarbide material having a large crystal grain size, i.e., larger than about 30 microns.
It is a still further object of this invention to provide a uranium dicarbide material having an oxygen content of less than about 400 parts per million.
Still another object of this invention is to provide a process of preparing uranium dicarbide materials in high theoretical density, preferably with large grain sizes, such a process also suitable for the preparation of other actinide carbide reactor fuels, such as uranium monocarbide, plutonium carbide, and plutonium nitride, and preparation of difficult to fabricate structural ceramics such as silicon, niobium, tantalum, tungsten, titanium, and zirconium carbides, such a process involving sintering a precursor pellet while in contact with either a carbon accepting material or a carbon donating material depending on whether the pellet is to lose or to gain carbon respectively.
Yet another object of the invention is to provide a densification process of preparing carbide materials wherein the density is varied in a controlled manner at different regions of, e.g. a pellet.